1,591 research outputs found
Competition between Phase Separation and Spin Density Wave or Charge Density Wave Order: Role of Long-Range Interactions
Recent studies of pairing and charge order in materials such as FeSe,
SrTiO, and 2H-NbSe have suggested that momentum dependence of the
electron-phonon coupling plays an important role in their properties. Initial
attempts to study Hamiltonians which either do not include or else truncate the
range of Coulomb repulsion have noted that the resulting spatial non-locality
of the electron-phonon interaction leads to a dominant tendency to phase
separation. Here we present Quantum Monte Carlo results for such models in
which we incorporate both on-site and intersite electron-electron interactions.
We show that these can stabilize phases in which the density is homogeneous and
determine the associated phase boundaries. As a consequence, the physics of
momentum dependent electron-phonon coupling can be determined outside of the
trivial phase separated regime.Comment: 9 pages, 7 figure
Supersolid phases in the one dimensional extended soft core Bosonic Hubbard model
We present results of Quantum Monte Carlo simulations for the soft core
extended bosonic Hubbard model in one dimension exhibiting the presence of
supersolid phases similar to those recently found in two dimensions. We find
that in one and two dimensions, the insulator-supersolid transition has dynamic
critical exponent z=2 whereas the first order insulator-superfluid transition
in two dimensions is replaced by a continuous transition with z=1 in one
dimension. We present evidence that this transition is in the
Kosterlitz-Thouless universality class and discuss the mechanism behind this
difference. The simultaneous presence of two types of quasi long range order
results in two soliton-like dips in the excitation spectrum.Comment: 4 pages, 5 figure
Superconducting Transitions in Flat Band Systems
The physics of strongly correlated quantum particles within a flat band was
originally explored as a route to itinerant ferromagnetism and, indeed, a
celebrated theorem by Lieb rigorously establishes that the ground state of the
repulsive Hubbard model on a bipartite lattice with unequal number of sites in
each sublattice must have nonzero spin S at half-filling. Recently, there has
been interest in Lieb geometries due to the possibility of novel topological
insulator, nematic, and Bose-Einstein condensed (BEC) phases. In this paper, we
extend the understanding of the attractive Hubbard model on the Lieb lattice by
using Determinant Quantum Monte Carlo to study real space charge and pair
correlation functions not addressed by the Lieb theorems
Metallic phase in the two-dimensional ionic Hubbard model
We investigate the phases of the ionic Hubbard model in a two-dimensional
square lattice using determinant quantum Monte Carlo (DQMC). At half-filling,
when the interaction strength or the staggered potential dominate we find Mott
and band insulators, respectively. When these two energies are of the same
order we find a metallic region. Charge and magnetic structure factors
demonstrate the presence of antiferromagnetism only in the Mott region,
although the externally imposed density modulation is present everywhere in the
phase diagram. Away from half-filling, other insulating phases are found.
Kinetic energy correlations do not give clear signals for the existence of a
bond-ordered phase
Transport and magnetic properties in YBaCo2O5.45: Focus on the high-temperature transition
The electronic transport properties and the magnetic susceptibility were
measured in detail in . Close to the so-called metal-insulator
transition, strong effects of resistance relaxation, a clear thermal hysteresis
and a sudden increase of the resistance noise are observed. This is likely due
to the first order character of the transition and to the underlying phases
coexistence. Despite these out of equilibrium features, a positive and linear
magneto-resistance is also observed, possibly linked to the heterogeneity of
the state. From a magnetic point of view, the paramagnetic to ordered magnetic
state transition is observed using non linear susceptibilty. This transition
shows the characteristics of a continuous transition, and time dependent
effects can be linked with the dynamics of magnetic domains in presence of
disorder. Thus, when focusing on the order of the transitions, the electronic
one and the magnetic one can not be directly associated.Comment: accepted for publication in PR
Exotic phases of interacting p-band bosons
We study a model of interacting bosons that occupy the first excited p-band
states of a two-dimensional optical lattice. In contrast to the much studied
single band Bose-Hubbard Hamiltonian, this more complex model allows for
non-trivial superfluid phases associated with condensation at non-zero momentum
and staggered order of the orbital angular momentum in addition to the
superfluid-Mott insulator transition. More specifically, we observe staggered
orbital angular momentum order in the Mott phase at commensurate filling and
superfluidity at all densities. We also observe a transition between the
staggered angular momentum superfluid phase and a striped superfluid, with an
alternation of the phase of the superfluid along one direction. The transition
between these two phases was observed in a recent experiment, which is then
qualitatively well described by our model.Comment: 8 pages, 12 figure
Rhodium Doped Manganites : Ferromagnetism and Metallicity
The possibility to induce ferromagnetism and insulator to metal transitions
in small A site cation manganites Ln_{1-x}Ca_xMnO_3 by rhodium doping is shown
for the first time. Colossal magnetoresistance (CMR) properties are evidenced
for a large compositional range (0.35 \leq x < 0.60). The ability of rhodium to
induce such properties is compared to the results obtained by chromium and
ruthenium doping. Models are proposed to explain this behavior.Comment: 11 pages, 8 figure
Exact Study of the 1D Boson Hubbard Model with a Superlattice Potential
We use Quantum Monte Carlo simulations and exact diagonalization to explore
the phase diagram of the Bose-Hubbard model with an additional superlattice
potential. We first analyze the properties of superfluid and insulating phases
present in the hard-core limit where an exact analytic treatment is possible
via the Jordan-Wigner transformation. The extension to finite on-site
interaction is achieved by means of quantum Monte Carlo simulations. We
determine insulator/superfluid phase diagrams as functions of the on-site
repulsive interaction, superlattice potential strength, and filling, finding
that insulators with fractional occupation numbers, which are present in the
hard-core case, extend deep into the soft-core region. Furthermore, at integer
fillings, we find that the competition between the on-site repulsion and the
superlattice potential can produce a phase transition between a Mott insulator
and a charge density wave insulator, with an intermediate superfluid phase. Our
results are relevant to the behavior of ultracold atoms in optical
superlattices which are beginning to be studied experimentally.Comment: 13 pages, 23 figure
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